The heads of the nation's military branches on Friday also were considering raising security levels at bases across the country.

"There is a large spike in threat reporting that shows al Qaeda cells are possibly close to attacks," a senior military official told CNN.

Also, state security officials said security will be increased at major bridges and tunnels. The officials said they would be calling authorities in the private sector and asking them to heighten security at key pieces of infrastructure such as nuclear power plants, railroad lines, and ports.

State Department officials told CNN that arrests of suspected terrorists in Britain with the chemical agent ricin, in France with cyanide compounds and in Spain with other chemicals prompted the caution.

The national threat levels, developed after the September 11 attacks, are part of a color-coded system. Currently the level stands at elevated risk -- or yellow. High risk would be orange.

Decision follows days of debate
Earlier sources told CNN that U.S. government officials had recommended to the president that he raise the threat level, following several days of debate within the government about whether to take such an action.

Counterterrorism officials have said they are more worried about a possible terrorist attack due to several factors, including an increase in the so-called "chatter" level.

Worldwide chatter prompted the State Department on Wednesday to issue a new worldwide warning that cautions Americans to be vigilant and aware of their surroundings. The chatter concerned possible a chemical or bioweapons terror attack against the United States.

The stepped-up chatter indicates a greater amount of intelligence coming in about a possible attack, especially by al Qaeda, the culmination beginning this weekend of the Hajj pilgrimage to Mecca, Saudi Arabia, and the continuing confrontation with Iraq, according to officials.

Ridge called governors
Sources said that Ridge had been calling state governors and emergency preparedness officials to brief them on the threat, telling them that the level of chatter was at its highest level since the terrorist attacks of September 11, 2001.

Ridge has also contacted officials in 13 sectors of private industry which control 85 percent of the critical infrastructure in the nation, including the energy, finance and transportation sectors. The officials said they were particularly concerned about chemical, biological and radiological threats.

On Wednesday, sources told CNN that the FBI is closely watching a "handful" of people believed to be Iraqi intelligence officers in the United States. There is also surveillance of at least several hundred Iraqi nationals living in the United States who are thought to be supporters of Iraqi President Saddam Hussein.

Also worrisome to U.S. officials is increased intelligence from a variety of sources in recent weeks suggesting a possible future attack in the United States that could include chemical or biological weapons.

The information does not single out any specific chemical agent or target, but officials said they are mostly concerned about the vulnerability of so-called "soft targets," such as apartment buildings, sports arenas and amusement parks.

Since the inception of the color-coded threat level system -- officially called the Homeland Security Advisory System -- in March 2002, the status has been yellow the whole time except for a short period around July 4 when it was raised to orange.